Native fish species as competitors in the diet of herbivorous aquaculture species in the reservoirs of the Volga-Don Shipping Canal

Due to the planned increase in aquaculture production in Russia, the effective use of existing water bodies, including the reservoirs of the Volga-Don Shipping Canal (Berezlavskoye, Varvarovskoye, and Karpovskoye), is of particular importance. In addition to their transport functions, these water bodies, with a total area of 84 km², have significant potential for increasing fish productivity through the introduction of plant-eating species such as white silver carp (Hypophthalmichthys molitrix) and white amur (Ctenopharyngodon idella). It has been established that the ichthyocenosis of reservoirs is formed mainly by limnophilic species, among which the dominant position in terms of relative abundance and biomass is occupied by roach (22.1%), as well as perch, crucian carp, and bream. An analysis of the contents of the gastrointestinal tracts of white silver carp revealed the dominance of microalgae in its diet (up to 70% by weight), with a pronounced seasonal succession of food objects: from diatoms in spring to cyanobacteria in summer-autumn. It is shown that potential trophic competition from the obligate planktivore, the blue whiting, due to the extremely low abundance of the latter (less than 1.3% in catches) in the pelagic zone of reservoirs, is not realized. A study of the trophic strategy of the white crucian carp demonstrated that its diet is based on submerged macrophytes (27.7% of soft and 16.3% of hard vegetation) and detritus (52.2%), with the latter playing a key role in the nutrition of juveniles, ensuring a high rate of somatic growth. It has been revealed that older age groups of roach, which are feeding in the coastal zone, consume higher aquatic vegetation in the amount of 15-23% of the mass of the food ball, which creates the prerequisites for the emergence of trophic competition with the white amur. The introduction of plant-eating fish into the reservoirs of the Volga-Don Shipping Canal is a biologically justified reclamation measure, but it requires further monitoring of the trophic interactions in the “grass carp – roach” system.